Solid or Liquid?

Scientists fully understand the process of water turning to ice. As the temperature cools, the movement of the water molecules slows. At 32 F, the molecules form crystal lattices, solidifying into ice. In contrast, the molecules of glasses do not crystallize. The movement of the glass molecules slows as temperature cools, but they never lock into crystal patterns. Instead, they jumble up and gradually become glassier, or more viscous. No one understands exactly why.

The Physics lab at Emory University began zeroing in on this question by designing a wedge-shaped chamber, using glue and glass microscope slides that allowed observation of single samples of glassy materials confined at decreasing diameters. For samples, the lab used mixtures of water and tiny plastic balls - each about the size of the nucleus of a cell. This model system acts like a glass when the particle concentration is increased.

The samples were packed into the wedge-shaped chambers, then placed in a confocal microscope, which digitally scanned cross-sections of the samples, creating up to 480 images per second. The result was three-dimensional digital movies, showing the movement and behavior of the particles over time, within different regions of the chamber.

The data showed that the narrower the sample chamber, the slower the particles moved and the closer they came to being glass-like. When the researchers increased the particle concentration in the samples, the confinement-induced slowing occurred at larger plate separations. The dimension between the plates at which the particles consistently slowed their movement was 20 particles across.

"It's like cars and traffic jams," says Eric Weeks, physicist at Emory University. "If you're on the highway and a few more cars get on, you don't really care because you can still move at the same speed. But at some point, your speed has to go from 40 mph to 5 mph. That's kind of what's happening with glass."

Previous research has shown groups of particles in dense suspensions move cooperatively. "Our work suggests glasses are solid-like because these groups can't move when the sample chamber is thinner than the typical size of these groups," Weeks says. "These experiments help us understand earlier work done with thin polymer films and other glassy materials, but as we use particles rather than atoms, we get to directly see how confinement influences the glass transition."